Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

JULY 2011

1. Northern Hemisphere

The 500-hPa circulation during July featured an alternating ridge-trough pattern that extended around the hemisphere. Regional aspects of this pattern included above average heights over the central North Pacific and North Atlantic Ocean, western Russia, and Mongolia, and below average heights over western North America, central Europe, and central Siberia (Fig. E9).

At 200-hPa, enhanced troughs over the tropical and subtropical central Pacific in both hemispheres reflected a lingering response to La Niña (Fig. T22).

The main surface temperature anomalies during July resembled the 500-hPa ridge-trough pattern, and included above-average temperatures across the eastern half of the U.S., western Russia, eastern Siberia, and central China (Fig. E1). Below average temperatures were observed in central Europe and central Siberia. The main precipitation signals included above-average totals across much of Europe, and below-average totals across the south-central and eastern U.S., and western Russia (Fig. E3).

a. North Pacific and North America

In the lower latitudes, the 200-hPa circulation during July featured amplified mid-Pacific troughs in both hemispheres (Fig. T22). This pattern is linked to the lingering La Niña-related pattern of tropical convection (Fig. T25).

In the extratropics, the mean 500-hPa circulation featured an amplified trough-ridge pattern extending across North America (Fig. E9). This pattern was associated with an amplification of the mean summer-time ridge normally situated east of the Rocky Mountains. This persistent ridge contributed to exceptionally warm surface temperatures across the eastern half of the U.S., with many regions recording departures in the upper 90^th percentile of occurrences (Fig. E1). The ridge also contributed to below-average precipitation across large portions of the eastern U.S., with the most significant deficits observed in Texas, the mid-Atlantic, and the Great Lakes region (Fig. E5).

The mid-Atlantic region has recorded below-average precipitation for much of the past year. However, much more severe precipitation deficits have accumulated across the southern U.S. during the past year. According to NOAA’s U.S. Drought Monitor, exceptional drought was evident during July across Texas, Oklahoma, southern Kansas, and portions of central Louisiana. Extreme drought was evident across New Mexico, southern Colorado, southern Georgia, and southeastern North Carolina.

b. North Atlantic and Eurasia

The 500-hPa circulation during July featured an alternating ridge-trough pattern extending from the central North Atlantic to eastern Siberia (Fig. E9). This pattern controlled the surface temperature (Fig. E1) and precipitation (Fig. E3) anomaly patterns throughout the region, with warmer and drier than average conditions associated with the mean ridges, and cooler and wetter conditions associated with the mean troughs. Specific regional anomalies included a cooler and wetter than average month in both Europe and central Russia, and warmer and drier than average month in western Russia, eastern Siberia, and China.

2. Southern Hemisphere  

In the subtropics, the upper-level (200-hPa) circulation during July reflected an amplified trough across the central South Pacific, and a reduced strength of the mean South Pacific jet stream (Figs. T21, T22). These conditions are consistent with the lingering La Niña-related pattern of tropical convection.

The 500-hPa circulation during July featured above average heights in the area south of Australia and Africa, and also across Antarctica, and below average heights south of New Zealand and over the central Indian Ocean (Fig. E15).  The most significant surface temperature (Fig. E1) and precipitation (Fig. E3) signals were observed in eastern Australia, where anomalously warm and dry conditions prevailed during the month.